Certain embodiments of the present invention are directed generally to the field of diagnostic ultrasound imaging. More particularly, certain embodiments of the present invention are directed to the field of ultrasonic cardiac monitoring systems and methods.
When a patient is given anesthesia, there is a risk of myocardial infarction when the patient is in the induction phase or is in the phase of being awakened. One of the first signs of myocardial infarction is a change in left ventricular wall motion. Cardiologists, anesthesiologists, and other medical personnel have a need for early detection of signs of changes in left ventricular wall motion.
Conventional methods of monitoring patients"" cardiac activity include watching the contraction of the left ventricle on an ultrasound diagnostic imaging system that obtains ultrasound data using a TEE probe lowered into the patient""s esophagus. Employment of a TEE probe lowered into the patient""s esophagus to monitor cardiac activity during surgery is potentially inaccurate because a physician has nothing to compare to the observed data and, therefore, the physician must memorize the previous functioning of the observed heart. Thus, if the difference between the current cardiac activity and previous cardiac activity is subtle, then the physician will not notice the change.
Another conventional method for monitoring cardiac activity involves using an ultrasound diagnostic imaging system to express general heart function using a single number representing the ejection fraction. The ejection fraction is the fractional change in the observed cross-sectional area of the left ventricle. Greater change in the cross-sectional area of the left ventricle over the cycle of a heartbeat indicates, better health of the heart. Physicians would like to observe changes in the single number representing ejection fraction because changes in that number may represent a significant change in the health of the heart. To obtain the number that represents ejection fraction, an algorithm is used to automatically trace the boundary of the endocardium by distinguishing between blood and heart tissue. Tracing is based on edge detection. Edge detection is performed by detecting sudden high video contrast between the heart muscle and the endocardium cavity, the former being displayed in shades of gray and the latter being displayed in black. Because tracing is based on edge detection, the operator may position a marker, using a track ball or other controller, in the left ventricle prior to tracing. Once the marker is positioned, the endocardium is automatically traced, identifying the inner border of the left ventricle. Tracing, however, may be lost from time to time if the image of the left ventricle is not of high quality because tracing will stop if the algorithm fails to detect the boundary between the endocardium cavity and heart tissue. Also, in some circumstances, tracing is not robust enough for obtaining an accurate number. When tracing is not very robust, tracing has too little contrast to obtain an accurate number to represent the ejection fraction.
Rather than using a TEE probe or edge detection to detect left ventricle motion, a Doppler method has also been employed to calculate cardiac output. Cardiac output is the volume of the blood that is ejected per unit time from the left ventricle when the left ventricle contracts. Cardiac output can be considered the volume of blood passing through the aorta per unit time, typically expressed in terms of liters per minute. Cardiac output is related to ejection fraction and heart rate. The Doppler method of detecting left ventricle motion requires probe manipulation by an operator, which increases the potential for human error. Measurements made by Doppler are proportional to the cosine of the angle formed between the directional velocity of the flow of blood and the beam direction. Because the angle formed between the directional velocity of the flow of blood and the beam direction changes over time, there is inaccuracy in the Doppler measurements. Also, the cross-sectional area of the vessel being analyzed must be measured, because the volume of blood passing through the vessel equals the cross-sectional area of the vessel multiplied by the velocity. But the cross-section of the vessel changes over time, because the vessel is elastic. Also, the angle from which the vessel is measured changes, changing the measured cross-section of the vessel. For example, the vessel must be measured from an angle perpendicular to the longitudinal axis of the vessel if the measurement is to accurately represent the actual vessel cross-section. If the vessel is not measured from an angle perpendicular to the longitudinal axis of the vessel, then the measurement will be the cross-sectional area of an ellipse, which is greater than the actual cross-sectional area of the vessel. Thus, the use of ultrasound measurements to determine cardiac output is an ineffective method of detecting the subtle changes in left ventricular wall motion that may precede a myocardial infarction.
One embodiment of the present invention is an apparatus for displaying a plurality of images in a diagnostic ultrasound system. The apparatus may comprise a display device having a first window for displaying a live ultrasound image and a second window for displaying a reference cine image. The images displayed in the first and second windows are synchronized by ECG gating. The images may be cardiac images. The reference image makes it easy for an operator to compare the current condition of the heart or other imaged region with the condition of that region at the time the reference image was captured.
The reference image may be captured and re-captured automatically at predetermined intervals. Alternatively, the reference image may be captured when the operator elects to capture the image, in which case the reference cine image is repeated until the operator elects to capture a new reference image. The reference cine image may be continuously updated so that the reference cine image lags behind the live ultrasound image by a pre-determined amount of time.
Alternative embodiments include a plurality of windows for displaying reference images. Embodiments having a plurality of windows for displaying reference images permit an operator to have a plurality of reference views of an area of interest that is displayed as a live ultrasound image in the first window.
A method in accordance with one embodiment of the present invention comprises the steps of displaying a live ultrasound image, displaying a reference cine image, and synchronizing, by ECG gating, the live ultrasound image and the reference cine image. The live ultrasound image and the reference cine image may be images of the heart. In a further embodiment, the method comprises the additional step of displaying a second reference cine image, wherein a view of the heart displayed in the first reference cine image is different from a view of the heart displayed in the second reference cine image. The reference cine image or images may be captured manually or may be captured and re-captured at a pre-determined interval. Alternatively, the reference cine image or images may be continuously updated so that the reference cine image or images lag behind the live ultrasound image by a pre-determined amount of time.